The stabilization efficiency of effluents from the Chirapatire wastewater stabilization plant demonstrated that the stabilization plant contributed significantly in improving the water quality which was vital to the wastewater-fed aquaculture success. This is because the solid and oxygen demand constituents of the wastewater which are mostly composed of organics and inorganics (Akhade et al., 2019) could cause clogging of fish gills and impairment of fish vision if not properly removed. These contaminants therefore affect fish feeding and their reproductive behaviour (Bunting, 2006). The high BOD and COD in the wastewater exceeding the EPA recommended levels could have directly impacted the low DO concentrations (Keraita et al., 2003). The stabilization efficiency of the COD (< 90%) and BOD (< 80%) demonstrated that the stabilization plant was effective in the removal of the organic and the inorganic constituents, and that the effluents were conducive for reuse in wastewater-fed aquaculture. COD and BOD constituents are mostly removed from wastewaters in stabilization plants via primary settling, filtration and sedimentation processes (Naidoo & Olaniran, 2014). BOD and COD represent the concentrations of both organics and in-organics and these parameters play important roles in the DO availability in wastewater (Akhade et al., 2019). The DO concentrations of > 1.0 mg/l in the effluents revealed that the effluent had a good potential of supporting wastewater-fed aquaculture. This is because DO concentration significantly affect the survival, the condition factor and the growth performance of fishes (Roosta & Hamidpour, 2011). Thus, DO is among the critical water qualities that affects the survival and growth performance of cultured fish, therefore to achieve optimum yield, it is important to maintain optimum water qualities throughout the production cycle (Masser et al., 1999). The problem was with the ammonia concentrations which were above the EPA standard. However, the African catfish (C. gariepinus) used in present study was able to thrive in such ammonia concentration because they have accessory organs that enable them to breathe atmospheric oxygen to nitrify the ammonia molecules (de Graaf & Janssen, 1996). The high ammonia concentration could have been the reason for the condition factors recorded at the end of the study (Ampofo & Clerk, 2003). Nevertheless, there was a higher fish yield and the contribution of stabilization ponds to food availability through primary production of phytoplankton bloom which substantially reflect a heterogeneity of production cost reduction could have been attributed to the fish yield recorded (Little et al., 2007). The fish yield recorded in present study might have been influenced by the phytoplankton bloom as direct food to the cultured fish sourced from the nutrients in the pond water and transformed into high-quality proteins (Ampofo & Clerk, 2003).
Fish harvested from the stabilization pond were discovered to have unsafe levels of the two microbes (total coliform and E. coli) and therefore required post-harvest treatments. Generally, the pathogen level of total coliform and E. coli in the tissues of the harvested fish were higher in the skin than the muscle and gut (ie in the order Skin > Gut > Muscle for both total coliform and E. coli). Results of pathogen levels of total coliform and E. coli in harvested fish tissues from the stabilization pond of present study is consistent with results from previous study (Yeboah-Agyepong et al., 2019) and this could be attributed to the direct contact of fish with pond water (Li et al., 2005).
Depurating the harvested fish with sterilized water for 16 days was effective in removing > 99% of the total coliform and E. coli contamination from fish tissues. Removal of pathogens from fish tissues during the fish depuration could be attributed to the action of the depurating water. When fishes were made to stay in the depurated unit for a 24-hour period, their movement allowed purging and cleansing before a complete discharge. Thus, while the depurating water could have provided disinfection to eliminate pathogens, the 24-hour complete water change could have also diluted and washed off the pathogens. No significant differences were recorded in pathogen removal between the two water volume treatments within the depuration days except on the 4th day which revealed a significant decrease (p < 0.05). This has demonstrated that it was possible to reduce the depuration water volume from full depurating unit volume (1500L1) to half depurating unit volume (750L) after the 4th day and still be able to achieve approximately the same percentage pathogen removal. This will help reduce the stress in the availability and the cost involved in procuring sterilized water for depurating contaminated fish. Results also revealed that it took longer days (approximately 8 days) for the removal of total coliform to attain > 90% in the muscle compared to the skin and the gut, which took lesser days (approximately 4 days) to attain > 90% pathogen removal. This could be due to the direct contact of the skin and the gut to the depurating water through swimming and passage of the depurating water through the alimentary canal. This is in support of Strauss's (1997) evidence, that reported that depuration of fish with accumulation of pathogens in tissues can be effective but depends largely on the pathogen load, surface exposure and duration. Pathogens in the muscles were believed to be removed through adsorption which takes longer than direct contact with the depurating water (Li et al., 2005). Also, during depuration, pathogens and contaminants in the gut were believed to be excreted as part of the digestive process, predominantly in the form of mucoid fecal strands (Li et al., 2005).
The microbial load on skin, muscle and gut tissues after smoking was significantly lower than the freshly harvested tissues. The total coliform and E. coli in fish tissues remained below recommended levels after a 60 minutes smoking period and the smoking of the fish was able to reduce pathogen loads significantly. Results in present study support previous evidence (Strauss, 1997) that reported that pathogenic risk could be effectively eliminated by heat application. While the temperatures from smoking has been proven to reduce bacteria growth in fish tissues, the smoke itself is a good pathogen inhibitor since it often contains bacteriocidal and antioxidant properties (Bryant, 2013). From the trends observed, 60 minutes was enough to effectively reduce the pathogen load to safe levels in the skin and muscles for human consumption, but was not enough for the gut. Results from the smoking duration also proved that, longer smoking duration yielded much percentage reduction on the pathogen removal than shorter duration. However, the economic aspect on the use of fuel to smoke at different duration was not considered in this study. Again, the cost of smoking versus depuration might be a determinant as to the acceptable measure required for controlling microbial load on fishes from waste water treatment plants.